A radar antenna is generally constructed so that its mechanical axis of symmetry is coincident with its electrical axis. In such an antenna, the radio frequency beam will then be substantially centered on the mechanical axis.
High gain and narrow beamwidth radar antennas often exhibit a "squint" situation in which the radio frequency beam being returned by the antenna will deviate slightly from the mechanical axis of the antenna. This "squint" often causes a problem when such an antenna is used in a high performance radar system and an attempt is made to calibrate a radar source with such target.
High gain point targets are often used to calibrate radar sources; these point targets frequently contain a corner reflector which is comprised of three reflecting plates oriented 90 degrees from each other.
However, the use of such high gain point targets often does not provide satisfactory calibration results. The corner reflectors in such point targets cannot exclude extraneous return signals within the radar antenna beamwidth at the same range as the corner reflector.
The corner reflectors must be placed on a high tower to keep the radar beam above the ground in order to minimize "skin return" and/or "clutter return" reflected from the ground. Sometimes the return from the tower and the ground will be significantly large, and the radar antenna will try to walk up and down the tower in elevation when fully locked onto by the radar. Furthermore, clutter also often is developed from objects nearby the target (such as buildings, trees, etc.) which can sum with the return from the corner reflector and can also corrupt the point target. This clutter then causes a "noisy" return and, when an attempt is made to align the radar source, tends to reduce the accuracy of the alignment.
It is an object of this invention to provide a radar target which provides a delayed return signal which is substantially free of spurious information, such as ground clutter.
It is another object of this invention to provide a radar target which provides a delayed return signal which can simulate objects at different distances than the target.
It is another object of this invention to provide a radar target which provides a delayed return wherein such delayed return can be made to appear to approach or recede incrementally at a rate coinciding with the Doppler frequency (velocity) imparted to the radar carrier frequency.
It is another object of this invention to provide a radar target adapted to return a signal to the radar transmitter which has substantially different characteristics than the signal received by the radar target.
It is another object of this invention to provide process for using the radar target of this invention so that the delayed radar signal returned to the radar source will contain virtually no clutter.
It is another object of this invention to provide a radar target system comprised of several targets above the radar horizon which is adapted to simulate multiple approaching threat aircraft at low altitude for use in training radar operators and for evaluating tactical battlefield radars.
It is another object of this invention to provide a process using two radar targets located at the ends of a runway to provide a runway centerline marker on the airport surveillance radar display.
It is another object of this invention to provide an "IFF" (identification friend or foe) radar target device to identify friendly vehicles or troops, wherein the target provides a coded (delayed) pulse group for identification.